First off I'm not trying to be a jerk in any way, but I think SmarterMail should stay out of the DNS business. I would never recommend using the existing SmarterMail DNS caching system as well. It's dangerous to cache any DNS record beyond its TTL. Those are stale records and shouldn't be used.
The default TTL most RBL / URIBL is 120 seconds. So if your DNS resolver simply honors the default TTL it would not query the same RBL / URIBL for the same query within the TTL of the record. They use a 120 second TTL for a reason... things change rapidly on the Internet. What was a good domain 5 minutes ago could be spewing out millions of spam messages right now (and vice versa).
If you WANT to cache records then you can do so if you run your own DNS resolver. You just set your server to force a minimum TTL to whatever length of time desired. I think it's a bad idea, but it's easy to do if you wanted to do it.
Again, I don't think that SmarterMail should be in the DNS business other than to properly query the DNS resolvers you define as the System Admin. Any DNS caching rules should be set up on your DNS resolver... not SmarterMail.
Our practice is that we honor TTL up to 4 hours. In other words even if a DNS query has a TTL of 86,400 seconds (a popular TTL that many use) we will only honor the TTL up to 14,400 seconds. Too many things change in a day... domains transfer, etc. and not everyone knows how to use DNS properly (or I should actually say there are few that know how to use DNS properly).